Numerous first transition metal oxide materials have been intensively investigated during the past decade for use as the positive electrode in rechargeable lithium batteries. These materials which may be classified as either lithiated or unlithiated metal oxides have been investigated because of their high gravimetric energy density.
Unlithiated first transition metal oxides include compounds such as V.sub.2 O.sub.5, V.sub.6 O.sub.13, TiO.sub.2 and MnO.sub.2. These materials may be coupled with negative materials to form an energy storage device such as a battery. The negative materials are limited, however, to active lithium containing materials such as metallic lithium and/or lithium alloys. Unfortunately, lithium and lithium alloys are not preferred for many applications because of their volatility in ambient conditions. Further, lithium poses many difficulties for electrode material processing and cell fabrication, since all the processes must be carried out in an inert environment.
Lithiated first transition metal oxides such as LiCoO.sub.2, LiMn.sub.2 O.sub.4, and LiNiO.sub.2, are positive electrode materials. These materials may be coupled with a negative electrode material to form a battery. Preferred negative electrode materials include metals such as Al, Bi, and Cd, and a lithium intercalation materials such as graphite. Metallic lithium and/or lithium alloys are not required as in unlithiated transition metal oxides. Accordingly, both the positive and negative electrodes can be processed and fabricated without inert environments. Therefore, lithiated metal oxides are more desirable as the positive material than unlithiated metal oxides.
Among lithiated first transition metal oxides, LiMn.sub.2 O.sub.4 is most attractive because it is least expensive, and is environmentally benign. Unfortunately, the gravimetric capacity of LiMn.sub.2 O.sub.4 is quite small having a theoretical capacity of only 148 mAh/g, and a practical capacity of typically less than about 120 mAh/g. Further, the high charge voltage necessary for materials such as LiMn.sub.2 O.sub.4 is near the potential at which electrolyte decomposition occurs. Accordingly, a slight overcharge may result in a significant electrolyte decomposition, and hence a significant decrease in battery performance.
Lithiated high capacity manganese oxides have been known for a number of years. For example, Li.sub.2 O stabilized, gamma--MnO.sub.2 can be electrochemically lithiated to form rechargeable LiMnO.sub.2 (as in the case of an unlithiated MnO.sub.2 discharged in a battery). This process is described in a pair of papers entitled "Preparation of High Surface Area EMD and Three-Volt Cathode of Manganese Oxide," by M. Yoshio presented at the IBA New Orleans Meeting of Oct. 9-10, 1993; and "Commercial Cells Based on MnO.sub.2 and MnO.sub.2 -Related Cathodes", by T. Nohma et al, in a publication entitled Lithium Batteries, edited by G. Pistoia, and published by Elsevier Press. These materials have been demonstrated to have a rechargeable capacity greater than 200 mAh/g at potentials higher than 2.5 volts but lower than 4 volts. Unfortunately, unlithiated manganese oxides as the positive material are less attractive than lithiated manganese oxides, as discussed earlier. Further, electrochemical lithiation as described in these papers is economically unfeasible at an industrial scale. Therefore, a chemical process for synthesis of a low voltage, high capacity lithiated manganese oxide is highly desirable. A lithiated three-volt manganese oxide which has the formula LiMnO.sub.2 is highly desirable as it possesses the advantages of both the existing four and three-volt manganese oxide. Such a material, prepared by an ion exchange method, was described by Ohzuku, et al in a publication entitled Chemistry Express, 7, 193 (1992). This material has been commented on extensively in subsequent papers such as the M. Yoshio paper referenced above, and in a publication entitled "Understanding the Disordered Structure LiMnO.sub.2 Prepared at Low Temperatures" by Reimers, et al. and appearing in the Journal Chemical Physics.
Unfortunately, the ion-exchange process described by Ohzuku, et al is both inefficient and does not lead to reproducible results, as the authors themselves acknowledge. Moreover, the ion-exchange process is not readily conducive to industrial scale manufacturing in commercial quantities.
Accordingly, there exists a need for a simple method by which to manufacture lithiated manganese oxide materials, such as LiMnO.sub.2. The manufacturing process should be relatively simple, take advantage of low-cost materials, and assure high repeatability and reproducibility of material characteristics fabricated accordingly to the process.